The present invention relates generally to ultrasonic transducers, and more particularly to a cylindrical or circular ultrasonic transducer designed to emit ultrasonic sound waves radially inward and to sense reflected ultrasonic waves from a pressure center at or near the center of the ultrasonic transducer. One particular application for the transducer of the present invention is in a gas analyzer that would be particularly useful in the medical field. A method of analyzing gas and a method of manufacturing the ultrasonic transducer are also disclosed.
Acoustical monitoring of gas can employ ultrasound having a frequency ranging from a few kHz to 10 MHZ. Acoustical techniques have been extensively used for gas flow monitoring. Most recently, efforts have turned to developing acoustical cells and processes which can determine the concentration of a component of a binary gas mixture. In general, acoustical concentration analysis of a gas mixture is performed by measuring the speed with which sound waves propagate through a gas mixture. Because the speed at which the sound waves travel through a gas is related to molecular weight, the concentration of a component of a gas mixture can be accurately determined.
Ultrasonic transducers that transmit and receive ultrasonic sound waves are common in the prior art, and have been commonly used in gas analyzers and gas flow meters. Typically, either two transducers are placed within a tube, or cylinder housing, one at each end, or a single transducer is placed at one end and an acoustical reflector at the other. For example, just a few include U.S. Pat. Nos. 5,452,621, 5,285,677, 4,938,066, 4,280,183, and 2,874,564, all of which either use multiple transducers, one for a transmitter and one for a receiver, or a single transducer and at least one separate reflector. This basic arrangement generally results in a low signal to noise ratio (i.e. high noise), an undesirable high detraction of emitted waves, and high structure reverberation. Much of the cited prior art attempts to correct these physical flaws of the transducers and the transducer arrangement through the use of sophisticated controls and processors.
It would therefore be desirable to eliminate these physical problems up-front at the source rather than at the back end with error correction controls.
Other types of methods and apparatus have been used as well for determining the concentration of a gas within a particular specimen, such as the concentration of oxygen as used in medical applications. For example, the concentration of oxygen has been detected by passing a gas over a fuel cell terminal that analyzes the gas being measured by generating a signal as a result of exposure of the gas to the cell terminal. Another, tests the consumption of oxygen in a test specimen and extrapolates that percentage to the overall gas sample. Another uses expendable chemical cells. Many others exist, but are generally unacceptable because they either interfere with the flow of the gas to be analyzed, have finite life due to cell consumption, require constant calibration, or are cost prohibitive.
Therefore, it would also be desirable to have an inwardly directed wave mode ultrasonic transducer, gas analyzer, and method of use and manufacture that solves all the aforementioned problems and could provide a high signal to noise ratio, has a wide emission surface area of for low defraction, and is relatively inexpensive to manufacture.